How to Insulate Your Home

Eco expert Nigel Griffiths looks at the key upgrade options that can help you improve your property’s thermal performance
Build It magazine expert Nigel Griffiths
by Nigel Griffiths
16th November 2020

For most people undertaking a major renovation project, upgrading the house’s energy efficiency will be a key goal. To get this right, you need to address each of the core components of the external envelope (floors, walls, windows and roof) and look at how their performance could be improved.

In this article I’ll explore the main options for upgrading each thermal element, but it’s important to take a holistic, whole-house approach so that everything works together.

Need to know: Green Homes Grant

The government has set out how the new Green Homes Grant, originally announced in July 2020, will work. Homeowners will only be eligible for money on secondary upgrades (such as double glazing and draught proofing) if they are carrying out so-called primary works (such as adding insulation and low-carbon heating).

Those on low incomes are eligible to claim 100% of the costs up to £10,000, whereas the majority of homeowners can access two-thirds of the price up to a total of £5,000.

You’ll need to account for the risks involved in altering a property’s structural fabric and be careful to avoid cold spots (which can cause condensation), along with making the necessary adjustments to heating systems and ensuring there’s adequate ventilation.

Without a proper assessment of the property, it’s impossible to predict exactly how much energy saving any particular insulation strategy will deliver. That’s because homes vary widely in terms of shape, construction, location, condition and services.

Even apparently homogenous buildings, such as those on terraced streets, may have been altered or extended in different ways. What’s more, patterns of occupation and energy use will also differ significantly. But the underlying principles remain the same. Here’s how to approach your eco upgrade.

Before you get started

Don’t upgrade insulation before you’ve dealt with basic upkeep. A well-maintained building will perform relatively efficiently, whereas a wet wall leaks energy fast – so it’s important to ensure that your property is dry and free from defects.

1. Insulating the roof

Heat rises, so a great deal of energy can be lost through an untreated attic void. If your home has a pitched roof and a flat ceiling below, leaving you with an unoccupied loft, it should be simple, cheap and cost-effective to add insulation.

Typically, you’ll be aiming to incorporate 270mm of mineral wool tucked between (and possibly over) the joists. There are a few things to bear in mind, however:

  • Don’t make the common mistake of forgetting to insulate the loft hatch.
  • Never block the eaves – in most cases, these need to be kept clear to allow for adequate ventilation and thus prevent condensation.
  • The loft void will move outside of the house’s thermal envelope, so you may need to insulate round cold water tanks to prevent freezing.
  •  A little insulation goes a long way, whereas it takes a lot to do a little more work. It might not be worth topping up if, say, you already have 150mm of mineral wool.
  • You may want to consider other products, such as sheep’s wool for its natural sustainability or options such as blown cellulose if the space is difficult to access.

If the roof has already been converted to offer living space, any thermal top-up would need to be integrated into the sloping ceilings – which can present more of a challenge. In most retrofits, insulation is added between the rafters, but it will be necessary to leave an air gap above this to allow for sufficient cross-ventilation.

Learn more: Insulating a New Roof

You can complement this with a layer below the rafters, but this will of course reduce head height. To minimise this effect, high-performance rigid foam insulation is normally used here.

2. Insulating ground floors

Solid ground floors are difficult – and sometimes impossible – to insulate. It’s not usually cost-effective to dig them out and add thermal protection beneath them, while adding insulation over the top will raise the floor level and is likely to cause issues at doorways and staircases. Instead, consider investing in a good quality underlay, which will do an effective job in most cases.

MeasureMain risksActionsLevel
Solid wall insulationHeritage impact
Moisture build-up
Indoor air quality
Thermal bridging
Assess condition of building
Use a whole-house approach
Employ qualified contractors
Cavity wall insulationMoisture build-upAssess location for suitability
Repair walls before injection
Monitor internal walls
Loft insulationMoisture riskEnsure ventilation at eaves
Check roof for condensation
Suspended floor insulationMoisture risk
Indoor air quality
Ventilate below floor
Check effect on ventilation within the building
Window upgradeHeritage impact
Thermal bridging at window reveals
Seek expert advice for historic windows
Use a whole-house approach
Improving air tightnessIndoor air quality
Moisture risk
Add ventilation if required
Monitor moisture levels
Table adapted and reproduced by permission of the Sustainable Traditional Buildings Alliance (

If there’s sufficient space and access, suspended timber ground floors can be treated from below. Alternatively, the floorboards can be lifted and insulation retrofitted between the joists from above.

This should be done very carefully if you’re dealing with an historic floor, as it may be an important part of the house’s character and value. It’s also essential to ensure that air bricks at all sides of the property are left clear and unobstructed, as this strategy reduces natural ventilation. If the sub-floor airflow is insufficient, you could end up with rotting joists.

3. Efficient windows

You might be surprised to learn that, often, more energy is lost through gaps around window frames than through the glass itself.

With that in mind, maintaining and upgrading existing units is always a solid plan. In many cases, existing frames can be repaired and retained, deeper casements inserted with double-glazed panels and new seals fitted.

Even adding a thick set of curtains – or opting for well-fitting shutters – will substantially reduce heat losses. If a single-glazed window has a U-value (a measure of heat loss, where lower figures indicate better performance) of 4.3 W/m2K, investing in heavy curtains could drop the figure to as low as 2.5 W/m2K, or even 1.7 W/m2K with well-fitting shutters.

Get a professional to advise on your cavity wall insulation
Professional installation, for example by a National Insulation Association member, is vital when retrofitting cavity wall insulation
Experts from Ventrolla draughtproof this window
A Ventrolla window renovation specialist installs draughtproofing into original sashes to aid airtightness

The most obvious solution can appear to be replacement – and in some cases it makes sense. This market has become saturated with PVCu double glazing, which to my mind is unfortunate. Timber versions are widely available and will preserve the look and value of a period property.

However, the majority of the benefits you’d get from making this switch could also be achieved through secondary glazing, which is not only much cheaper than replacement but involves a far lower environmental impact.

The technique means you can also preserve the historic single-glazed cylinder glass, with all its attractive imperfections, whilst simultaneously improving performance (U-values can reach as low as 1.8 W/m2K or even better if combined with other measures).

If your property has already been fitted with early double glazing, it’s unlikely to be as efficient as units manufactured to current regulations. Nevertheless, in most cases the in-use savings made by replacing this with newer double or triple glazed versions would be outweighed by the energy and cost of replacement.

4. Insulating walls

There are three main classes of walling in the UK housing stock, and the period of your property can help to indicate which one is likely to have been used.

The dates given below are approximate – and you should always investigate your property’s makeup thoroughly before you undertake any upgrades – but they set out a worthwhile general overview:

  • Solid (single leaf) wall construction – brick or stone – pre 1919
  • Cavity walls, uninsulated – brick and block – 1920-1975
  • Cavity walls, insulated – brick and block – post 1975

Solid walls

These can be insulated inside or out. Either way, it’s an expensive process and the benefits need to be carefully weighed up, especially where an elevation contains a high proportion of windows or doors.

External wall insulation (EWI) has the advantage that the original masonry becomes a heat store, able to absorb and radiate warmth back into the living space – so you get a very thermally effective result. The method adds a new weathering layer to the outside of the property, and it’s essential this is accurately detailed.

Sealants should be avoided, as these break down over time. It’s also a good idea to use moisture-open insulation products, such as sheepswool, as otherwise any water that enters the wall matrix will become trapped, which will lead to damp internal walls.

A key consideration is that, with period homes, it may be essential to retain the appearance of the original building – which may make it difficult to use EWI. It could, however, still be considered on some rear elevations or on properties that have been rendered later in their lifetime.

Internal wall insulation (IWI), by comparison, is not quite so effective, as it reduces the masonry’s ability to store useful heat. It will also reduce internal floor area slightly – plus there’s still potential to impact on the house’s heritage, as original skirting boards and cornicing are difficult to retain or replicate accurately. You’ll also need to remove and reinstate any services currently in place, such as radiators and power outlets.

Applying IWI will mean the original masonry walls become colder, which in turn reduces their ability to deal with moisture ingress. It’s therefore essential to ensure the building is weathertight before installation.

Open plan white living rom
When George and Rosie renovated this Victorian house, experts recommended they prioritised insulation and airtightness as essential elements
Renovation & Extension of a Period Home
Consequently, they opted for a range of insulation, including Kingspan's rigid boards. Click on the image to read the full story

You’ll need an independent expert to assess the risk of condensation and mould growth at the interface between the IWI and the walling – and, again, you should always specify moisture-open products.

Bear in mind, too, that there will be thermal bridging (where heat escapes across a structural element that has a direct connection between inside and out) at locations where the IWI can’t be applied. This could include the likes of staircases abutting external walls and suspended upper floor matrixes. Condensation is more likely to occur at these points.

Cavity walls

Up until the mid 1970s, cavity walls were constructed without insulation, as the outer layer was simply viewed as protection from the elements. It’s now common to retrofit insulation by injecting it into the cavity – a relatively cheap and simple process, with no disruption to the internal space.

However, in recent years there has been an increase in the failure rate of this kind of cavity wall insulation (CWI). This is because the gaps between masonry leafs were designed to be ventilated – and retrofit CWI prevents this.

If there are any imperfections in the external skin, such as cracked render, water will enter and become trapped. It will then be conveyed to the inner skin, resulting in mould growth where previously there was none.

In such cases, the insulation will have to be removed by professionals, which is an expensive job – so it’s important the installation is completed correctly. For the same reasons, in areas of high exposure to driven rain, it simply may not be appropriate to fit CWI.

Later cavity walls already have insulation built-in, although there may not be a great amount of it. Yet even where there’s space to add more, it’s usually pointless to do so, as the cost and impact of the work is likely to outweigh the minimal improvements in energy savings.

Bear in mind that retrofitting any wall insulation will reduce natural ventilation to some degree – so the adequacy of a fresh supply needs to be checked afterwards to ensure good indoor air quality.

5. Improving airtightness

This is often overlooked as a means of saving energy. In an unaltered building of traditional construction, air leaks through gaps between floorboards, floor/wall junctions, sash windows, services penetrations, open chimneys etc. All told, this can amount to 30% or more of heat losses.

Many of these air pathways are simple and cheap to seal up – for instance by replacing seals around window frames, closing chimneys (leaving through ventilation by means of an air brick) or using a good quality underlay on butt-jointed timber ground floors.

Sash windows can be upgraded by inserting draught proofing strips to seal the gaps between the casement and the frame. This kind of restoration work is normally carried out by specialists.

However, we all need fresh air – and natural (sometimes called uncontrolled) ventilation is an important way of keeping older homes dry. After 100 years or more, buildings inevitably develop some weaknesses. Many will be admitting water through faults in masonry, missing or broken seals, incorrect ground levels and – most commonly – defective rainwater goods.

So when improving airtightness, it’s essential to retain sufficient ventilation to provide decent indoor air quality and help the building deal with any moisture ingress.

Top image: Ecoroll from Thermafleece is a natural sheepswool insulation, suitable for use in walls, floors and roofs.

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